Many vehicle members such as vehicle chassis members and body structure members have heretofore been manufactured by pressing steel sheets having a prescribed strength. From the viewpoint of preserving the global environment, there has recently been a great need for vehicle bodies to be reduced in weight. Efforts are being made to strengthen steel sheets that are used, thereby reducing the thickness of the sheets. However, steel sheets deteriorate in terms of press formability as the strength of the steel sheets increases. Thus, it is becoming increasingly difficult to fabricate a steel sheet into a shape of a desired member.
Great Britain Patent No. 1490535 proposes a processing technique called hot pressing, in which a steel sheet that has been heated is deformed with a mold including a die and a punch and is simultaneously cooled rapidly, thus enabling easy processing and achieving an increase in strength. However, the hot pressing technique has a problem in that because a steel sheet is heated to a high temperature of about 950° C. before being hot pressed, scales (iron oxide) are formed on the surface of the steel sheet and come off during hot pressing to damage the mold or roughen the surface of the hot pressed member. Further, scales remaining on the surface of the member can make the appearance unsatisfactory or can lower the adhesion to a coating. To remedy such problems, scales on the surface of members are usually removed by performing a treatment such as acid cleaning or shot blasting. Because of the need for such a treatment, however, production steps become complicated and productivity is lowered. Although vehicle members such as vehicle chassis members and body structure members are required to be excellent in terms of corrosion resistance, hot pressed members manufactured through steps such as those described above are very poor in terms of corrosion resistance because they are not provided with any rust preventive layers such as deposited coatings.
To cope with such problems, a hot pressing technique is desired by which scales can be prevented from being generated during heating before hot pressing and the corrosion resistance of hot pressed members can be improved. With regard to this, there have been proposed steel sheets having a coating such as a deposited coating layer on the surface, as well as hot pressing processes using such steel sheets. For example, Japanese Patent No. 3663145 discloses a process for the manufacture of a hot pressed member with excellent corrosion resistance, in which a steel sheet coated with Zn or a Zn-based alloy is hot pressed such that a Zn—Fe-based compound or a Zn—Fe—Al-based compound is formed on the surface of the product. Japanese Patent No. 4039548 discloses a hot pressed product (member) with excellent formability, weldability and corrosion resistance that is obtained by heating a galvannealed steel sheet at 700 to 1000° C. for 2 to 20 minutes and hot pressing the steel sheet to form a coating layer containing a Fe—Zn solid solution phase on the surface of the product.
Hot pressed members manufactured according to the process of JP '145, however, have a defective appearance and exhibit a low adhesion to a coating due to the occurrence of scales. Further, the corrosion resistance can be degraded as a result of the formation of ZnO. Furthermore, hot pressed members described in JP '145 and JP '548 have poor resistance to corrosion perforation that is apt to be caused at a region in which a chemical conversion coating or an electrodeposition coating cannot be deposited with high throwing power (hereinafter, referred to as corrosion perforation resistance).
Further, another hot pressing technique has been recently examined in which a steel sheet is cold pressed into a desired shape prior to hot pressing. Accordingly, hot-pressing steel sheets have been increasingly required to exhibit excellent cold pressing properties.
It could therefore be helpful to provide a hot-pressing steel sheet excellent in oxidation resistance to be capable of suppressing the formation of scales and ZnO during hot pressing as well as is excellent in cold pressing properties, and to provide a process for manufacturing a hot pressed member using the steel sheet.
It could also be helpful to provide a hot-pressing steel sheet which is excellent in corrosion perforation resistance after being hot pressed as well as is excellent in cold pressing properties, and to provide a process for manufacturing a hot pressed member using the steel sheet.